/Engine/Source/Runtime/Core/Public/Misc/CoreDelegates.h
IOS-style application lifecycle delegates This is called when the application is about to be deactivated (e.g., due to a phone call or SMS or the sleep button). The game should be paused if possible, etc...
static [TMulticastDelegate](API\Runtime\Core\Delegates\TMulticastDelegate)< void()> ApplicationWillDeactivateDelegate;